When it comes to vaccination, bad news is contagious

And on Twitter, good information apparently does not bear repeating.

In recent years, the controversy about vaccine safety has exploded online. Fueled by pseudoscience, conspiracy theories, and ignorance, a surprisingly large number of people today refuse to vaccinate themselves or their children. According to a 2011 poll, nearly a quarter of Americans have changed their opinion on vaccination in the last five years, and for the vast majority of these people, that change has been in a negative direction.

There’s no better tool than social media to spread information—and misinformation—about controversial topics quickly and efficiently, and there has been speculation that these outlets have played a role in heightening concerns about vaccination. But how do opinions about health-related behaviors spread over social media?

A group of researchers from Penn State used Twitter to answer this question, and their findings were published in EPJ Data Science last week.

During the H1N1 epidemic in 2009, the researchers gathered hundreds of thousands of tweets containing key words like “vaccine” and “vaccination." From this set, they eliminated all tweets that were not related to the H1N1 vaccine. They then recruited Penn State students to judge whether each of a subset of these 318,379 tweets expressed a positive sentiment about the vaccine (such as “Getting the flu vaccine today!”), a negative sentiment about the vaccine (like “Just found out that the H1N1 vaccine causes nerve disease”), or was neutral toward the vaccine. Based on these decisions, a computer algorithm was used to classify the rest of the tweets.

The researchers then modeled the likelihood that a given Twitter user would tweet either a positive or negative opinion about the H1N1 vaccine. The model included various predictors, such as how many positive or negative vaccine-related tweets users saw, the number of people they followed, and whether these people followed them back.

The results were unexpected, and, frankly, somewhat scary from a public health perspective. First, it was clear that negative information is extremely contagious: the more negative tweets a person received about the H1N1 vaccine, the more likely they were to tweet a negative sentiment about vaccination. Oddly, this finding didn’t hold for positive sentiments. In fact, the opposite was true: a high volume of positive tweets about the vaccine also increased the likelihood of negative vaccine-related tweets.

A similar contradiction was found when it came to reciprocal Twitter relationships, in which users follow each others’ tweets. Users who had many reciprocal connections with people who tweeted anti-vaccine sentiments also tended to express a lot of negative opinions about the H1N1 vaccine. But users who had many reciprocal connections with pro-vaccine users were not particularly likely to tweet positive opinions about the vaccine.

So why the asymmetry? If tweets bashing vaccines are so socially contagious, why don't tweets that extol the virtues of vaccines spread quickly as well? Unfortunately, the researchers don’t know yet. It’s possible that there’s something about the way negative opinions are expressed on social media that makes them contagious, or it could be that scary information is especially likely to be passed on.

There are, of course, a few caveats to the findings. The study could only assess sentiments rather than actual behavior when it comes to vaccinations; social pressure might cause people to tweet negatively about vaccines, even though they themselves did get vaccinated against H1N1 (or vice versa). Furthermore, the research couldn't capture any information about people who didn’t tweet about vaccinations during the study period, or those who don’t tweet at all.

But if future studies confirm that exposure to positive messages actually increases the incidence of negative messages—at least when it comes to vaccination—public health strategies may need to be revamped. Rather than flooding social media with vaccine-positive information, a better strategy may be to try to limit the spread of negative information about vaccination, or at least try to address what's already out there.

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I work in local public health communication, and I think I can give you at least one reason for the asymmetry here. It's the same reason why it's so difficult to get almost any public health message through to the masses: It's very, very tough to prove and show what does not happen. A firefighter can prove he saved a life when he pulls a person from a burning building. Preventing disease or death that never occurs in the first place because of a vaccine is much, much harder.

Something that doesn't happen isn't worth talking about for most people. But negative effects - or the myth of negative effects - is much juicier to talk about.

310 Reader Comments

If tweets bashing vaccines are so socially contagious, why don't tweets that extol the virtues of vaccines spread quickly as well? Unfortunately, the researchers don’t know yet. It’s possible that there’s something about the way negative opinions are expressed on social media that makes them contagious, or it could be that scary information is especially likely to be passed on.

or maybe it's just that no one wants to argue with crackpots because no good ever comes from that.

Wow, it is true! When an organism reaches a certain critical population, genes can switch on in individual members to drive destructive behavior, thereby reducing the population and improving the quality of life for healthier organisms! WOOHOO! /sarcasm

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Seriously, I actually use the cognitive dissonance of the antivaxxers main spokesperson in a statistics class:

"Vaccines are POISON!" She claims while injecting her face with one of the most deadly neurotoxins known (botox).

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

This is America, the land of conspiracy. If someone says something plausible, but then is proven wrong by the people in charge, it must be because the people in charge are part of some global left/right/up/down network of evil doers.

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

I understand the frustration, and I don't know how to make your choice easier, but be glad you have the foresight to protect your children. Though I am glad I just got my TDAP!

By the way, does anyone know at what point the herd immunity things stop? Aren't their some vaccines that were stopped because of herd immunity? And if so, how do we go about getting those? Can adults get them?

"According to a 2011 poll, nearly a quarter of Americans have changed their opinion on vaccination in the last five years, and for the vast majority of these people, that change has been in a negative direction."

That's totally fine, that means more vaccines for me! Then when I'm healthy, and others get sick, I can laugh at them.

It's win-win, really.

Not really, some vaccines cannot be given to small children and therefor rely on herd immunity. That is, since you are immune thru vaccination, you are unlikely to pass it onto someone else who doesn't have immunity, such as a small child who can't be vaccinated for measles until 2 years old or so. So stupid ignorant people do have an impact on those too innocent to know better yet, and there are no laws out there allowing you to find out who is unvaccinated around your children so you can make an informed choice as to where I can take a young child. Therefore there are instances where lack of vaccination of one person is deliberately putting at risk other people. Ignorant behavior that causes reckless endangerment, and in things like driving and firearms, that's a crime.

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

If your child is vaccinated, isn't it relatively safe to let them play with non-vaccinated children? Not as good, mind you, but on a scale of "no risk" to "tons of risk", it's on the no risk end.

I work in local public health communication, and I think I can give you at least one reason for the asymmetry here. It's the same reason why it's so difficult to get almost any public health message through to the masses: It's very, very tough to prove and show what does not happen. A firefighter can prove he saved a life when he pulls a person from a burning building. Preventing disease or death that never occurs in the first place because of a vaccine is much, much harder.

Something that doesn't happen isn't worth talking about for most people. But negative effects - or the myth of negative effects - is much juicier to talk about.

"According to a 2011 poll, nearly a quarter of Americans have changed their opinion on vaccination in the last five years, and for the vast majority of these people, that change has been in a negative direction."

That's totally fine, that means more vaccines for me! Then when I'm healthy, and others get sick, I can laugh at them.

It's win-win, really.

Except that the way most vaccines are effective is through herd-immunity, so your vaccine won't help you, for example, not get polio when fewer than 90% of people the get polio vaccine.

A relatively few people falsely believing they don't need to be vaccinated can literally hurt everyone. That's why there are requirements for public school, for example.

It's that damn hindbrain. Fear isn't rational; it's an emotional response to a negative situation. We remember what scares us most so that we can avoid it in the future. Neutral and even positive events can be allowed to slip through the cracks since repeating them isn't harmful. So, despite the fact that all of the anti-vaccination rhetoric can be, and has been, logically and scientifically countered, it's going up against a few million years of hardwired survival instinct.

It would be really helpful if the people decrying vaccinations would remember that they themselves are likely thoroughly vaccinated.

Few things anger me more in this day and age then the continuation of these vaccination conspiracy nutjobs.

They live in a country affluent enough to make available these medical marvels to the populace at large. But instead of embracing such simple life saving tools, they actively neglect them and even worse they try and convince others to as well.

So frustrating. I'm pro-vaccine for children. My wife believes the hype and is anti. We don't have kids yet, so we've postponed the argument, but still frustrating. Not looking forward to revisiting that when the time comes. Apparently some people have forgotten about polio, thus the complacency.

That being said, I haven't had a vaccine in ages, but it's not due to conspiracies, but rather lack of making it a priority. Then again, I get pretty sick about every year too, so there's that. Back when I was in the military and got every shot in the book on a regular basis, I had a good 6 year run without getting sick outside of allergies. I'm almost positive I was a guinea pig for pharmaceutical companies in bed with the govt, but it WAS awesome being super healthy for so long.

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

If your child is vaccinated, isn't it relatively safe to let them play with non-vaccinated children? Not as good, mind you, but on a scale of "no risk" to "tons of risk", it's on the no risk end.

I was thinking this same thing - isn't the point of vaccination such that your kids could be exposed to the disease and not contract it?

I thought one of the public health hazards of not vaccinating is allowing a disease to progress and possibly mutate such that our existing vaccines would become outdated, yes? As well as the higher cost of treatment instead of prevention which means all of us have to pay for it (either directly through free/written off medical care or indirectly through higher insurance premiums).

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

If your child is vaccinated, isn't it relatively safe to let them play with non-vaccinated children? Not as good, mind you, but on a scale of "no risk" to "tons of risk", it's on the no risk end.

I was thinking this same thing - isn't the point of vaccination such that your kids could be exposed to the disease and not contract it?

I thought one of the public health hazards of not vaccinating is allowing a disease to progress and possibly mutate such that our existing vaccines would become outdated, yes? As well as the higher cost of treatment instead of prevention which means all of us have to pay for it (either directly through free/written off medical care or indirectly through higher insurance premiums).

There's still the risk of the loss of herd immunity. There are some diseases we no longer vaccine against anymore simply because they've been effectively wiped out. So even those that don't have a vaccination against said disease are still by all counts "vaccinated" as all of the people around them are either vaccinated or also clear of said disease.

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

I understand the frustration, and I don't know how to make your choice easier, but be glad you have the foresight to protect your children. Though I am glad I just got my TDAP!

By the way, does anyone know at what point the herd immunity things stop? Aren't their some vaccines that were stopped because of herd immunity? And if so, how do we go about getting those? Can adults get them?

Herd immunity works effectively when vaccination levels are above the 85-90% range, it is the mechanism which keeps the remaining 10% (which could, fairly, be comprised of crackpots or those who legitimately should not be vaccinated for other medical reasons) safe.

Sadly several family members of mine are anti-vaccination. This means that should I get around to having children I will not be able to permit my children to play with theirs. Its a frustrating situation.

Just yesterday I was having a "discussion" with an anti-vaccine friend on FB. Their entire argument is basically that "if vaccines work then my un-vaccinated children are a risk to your vaccinated children" I tried to explain herd immunity, that vaccines are 100% and the whole thing about organisms possibly mutating into a novel form but it was all lost on them.

Tangential to the vax/anti-vax discussion I'm surprised to see that this article has none of the lengthy psuedo-science filled comments demanding that we "discuss the controversy" that have appeared in the comments for the climate change article posted on Ars within the past day. How is it that Ars readers accept the science of vaccination but still dispute climate change?

Not a scientific opinion here, but a philosophical one: When one cannot use science to support one's views, one must resort to rhetoric.

Those who trust the science, there's nothing to argue about, no convincing, it's a simple set of facts (and the real difference between truth and fact is whether people argue about it). For people who don't trust the science, they can't use science to convince, they must use rhetoric. Hence they spend a lot of time talking, cajoling, coming up with arguments that are pity or rhyme. Hence....twitter/facebook.

I see someone doesn't know what the world looked like before vaccines. Infectious diseases were rampant. Even the simplest of diseases killed a lot of people. Peoples are destined to make the same mistakes over and over, because they are ignorant of history that preceded their lifespans, and sometimes ignorant of significantly newer history. Let's say anything that is no longer being featured in mainstream media. It doesn't exist, right? Never happened.

While these people are obviously wrong, could it be a good thing that there's a small contingent not getting vaccinated? Please correct me if I'm mistaken/conflating things, but I thought complete immunity from one variant of a virus means a stronger variant would survive and spread, thus making that strain hard or near impossible to cure. Is there a good balance between herd immunity and superbug prevention? Somebody who knows more about this please help.

Obviously this doesn't apply for things like polio, but what about things like seasonal flus? (I really don't know much about this at all, sorry for how stupid this probably is)

I have a 1 year old and just went through this with the wife. She was apt to listen to the warnings that are everywhere, especially here in the San Francisco area. Its the affluent that tend not to vaccinate their kids. Young, "educated", white suburban mom's.

After a lot of reading, I pushed us to vaccinate our child and get the whooping cough vaccine ourselves. But, I am not one to get a flu shot every autumn and I do think there is ample motivation for Big Pharma to pedal unnecessary vaccines for profit.

There are studies from Sweden, Finland, Denmark that do show that the adjuvant used in the H1N1 vaccine distributed in the European Union, Pandemrix, increases the likeliness of narcolepsy for children by around 15%. It's still a small number, but considering the number of people who received the vaccine, this means thousands of additional cases across Europe, and possibly Canada and Brazil since a very similar vaccine was used there. (No influenza vaccine with adjuvants are authorized in the US, so you guys are safe.)

There's no doubt that vaccines, in general, are good for people. But since they're distributed on such a wide scale, any undesirable side effect is sure to affect a lot of people, and this is undoubtedly a problem when they have to be authorized in a hurry. Most vaccines distributed to newborns have been tested for several decades now and there's no reason to fear them, but vaccines for episodic diseases don't go through that much.

Few things anger me more in this day and age then the continuation of these vaccination conspiracy nutjobs.

They live in a country affluent enough to make available these medical marvels to the populace at large. But instead of embracing such simple life saving tools, they actively neglect them and even worse they try and convince others to as well.

It just makes my blood boil.

It makes an interesting case study of natural selection, though. Whether knowingly or not they are increasing the likelihood that their contribution to the gene pool dead ends with their (dead) children.

The unfortunate side effect is that the children who are allergic to vaccines lose the benefit of herd immunity and also die, but that's arguably a good thing. (Children dying through no fault of their parents or their own is not a good thing, obviously, but from a species survival perspective the elimination of poor mutations is a positive trend.)

Negative vaccine reactions are very noticeable, but nothing obvious happens when a vaccine protects us from getting a disease. If there was a flashing sign over you that your vaccine had just protected you from getting disease X then it would give you something to tweet about. "Wow, I didn't get polio today because my vaccine protected me! This is awesome. I can't believe anyone would skip this vaccine!". Everyone has probably either had an arm that was sore for a week after a vaccination or at least heard of someone else complaining about it. Vaccine reactions worse than this, although uncommon, do occur. When someone does have a bad vaccine reaction they tend to let everyone know about it. Also, if someone has something happen to them just after getting a vaccination they will often associate the two, since people are very quick to make associations between two events that may or may not be related.

There's also the evolutionary mechanisms in place that cause people to be extra cautious of anything that could possibly be dangerous. Running away because a rustling in the bushes could be a lion doesn't have significant negative consequences if it's just the wind. Avoiding vaccines certainly might.

Vaccines for seasonals like the flu are IMO stupid, unless you are in a particularly exposed segment (elderly, immune system deficiency, etc.). But the rest - the vaccines for the really dangerous diseases? They should be mandatory. I don't care about freedom of choice, when that choice puts others (and more importantly my children) at risk. It's reckless endangerment - plain and simple.

The findings aren't confusing at all. The internet simply brings out the troll in all of us.

Its also a case of major vs minority opinions. And on the internet the minority opinions ring out the loudest, and cause the most furor. And since twitter is basically a one-liner messaging board contest, the minority opinions are even louder.

If a more in depth study was done I would bet that, those that look to twitter for 'news' also don't tweet majority opinions they agree with as much as those they disagree with. And those that do tweet majority opinions on are subjects that the user has a big personal connection with.

Negative vaccine reactions are very noticeable, but nothing obvious happens when a vaccine protects us from getting a disease. If there was a flashing sign over you that your vaccine had just protected you from getting disease X then it would give you something to tweet about. "Wow, I didn't get polio today because my vaccine protected me! This is awesome. I can't believe anyone would skip this vaccine!"

It's that damn hindbrain. Fear isn't rational; it's an emotional response to a negative situation. We remember what scares us most so that we can avoid it in the future. Neutral and even positive events can be allowed to slip through the cracks since repeating them isn't harmful. So, despite the fact that all of the anti-vaccination rhetoric can be, and has been, logically and scientifically countered, it's going up against a few million years of hardwired survival instinct.

It would be really helpful if the people decrying vaccinations would remember that they themselves are likely thoroughly vaccinated.

People will sit in traffic, suck down fumes all day, eat preservatives for years, eat all sorts of nasty shit for decades straight, smoke, drink - then say "hmm, that one time 50 mcg dose of vaccine is definitely the cause of something bad".

So frustrating. I'm pro-vaccine for children. My wife believes the hype and is anti. We don't have kids yet, so we've postponed the argument, but still frustrating. Not looking forward to revisiting that when the time comes. Apparently some people have forgotten about polio, thus the complacency.

That being said, I haven't had a vaccine in ages, but it's not due to conspiracies, but rather lack of making it a priority. Then again, I get pretty sick about every year too, so there's that. Back when I was in the military and got every shot in the book on a regular basis, I had a good 6 year run without getting sick outside of allergies. I'm almost positive I was a guinea pig for pharmaceutical companies in bed with the govt, but it WAS awesome being super healthy for so long.

I would agree with being pro vaccine for kids. Though, I fall on the opposite extreme, I haven't had a vaccine since I was in grade school (around 20 years ago), and I haven't had the flu in almost 15 years, and have only had a cold 3 times in that amount of time. I also don't sanitize my hands (I do wash them when going to the restroom, cleaning chicken, ect, though not with anti-bacterial soap)... I don't know how that would affect others around me, but the people I work closely with don't get sick very often, either, so I'm not sure if they get the shots or not. So, I guess I'm saying I don't get vaccines, rarely get sick, and don't know if that helps or hinders other people's attempts to stay healthy...

Vaccines for seasonals like the flu are IMO stupid, unless you are in a particularly exposed segment (elderly, immune system deficiency, etc.). But the rest - the vaccines for the really dangerous diseases? They should be mandatory. I don't care about freedom of choice, when that choice puts others (and more importantly my children) at risk. It's reckless endangerment - plain and simple.

I always have to weigh my decision carefully when flu season rolls around. I've gotten the flu vaccine about 50% of the years that i've been an adult, and every time I get serious flu symptoms for 4 days or more.

I have a 1 year old and just went through this with the wife. She was apt to listen to the warnings that are everywhere, especially here in the San Francisco area. Its the affluent that tend not to vaccinate their kids. Young, "educated", white suburban mom's.

After a lot of reading, I pushed us to vaccinate our child and get the whooping cough vaccine ourselves. But, I am not one to get a flu shot every autumn and I do think there is ample motivation for Big Pharma to pedal unnecessary vaccines for profit.

As someone who takes mass transit, it wasn't until I started getting the vaccines that I stopped getting the flu. Before then, it was an annual thing for me. If you're around a lot of people, things we all grab onto, etc., get your flu vaccine and let the knuckleheads suffer.

Two principal causes for the spread of negative "information" about vaccines, both fueled by ignorance:

(1) individuals try to demonstrate that they're not chumps, so they spread negative disinformation;

(2) cognitive dissonance created by the scientific fraud of one scientist who made $750K testifying as an expert witness that childhood vaccines cause autism. We still live (and many people have no doubt died) in the aftermath of this jerk.

So frustrating. I'm pro-vaccine for children. My wife believes the hype and is anti. We don't have kids yet, so we've postponed the argument, but still frustrating. Not looking forward to revisiting that when the time comes. Apparently some people have forgotten about polio, thus the complacency.

You should do what is best for your children medically. Which is, almost certainly, what your doctor says to do.

You could also lose the genetic lottery and have a kid who can't handle some or all vaccines. In which case he'll depend on herd immunity, which is declining because of this anti-vax business.

Kate Shaw Yoshida / Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas.